In patients with reduced or no renal function at all, waste products including toxic substances are removed by a kidney replacement therapy, wherein the blood of the patient is fed from the patient to the artificial kidney, respectively the dialyzer, by a blood supply conduit. Inside the artificial kidney, respectively the dialyzer, the blood of the patient is brought into contact with the dialysis fluid via a semi-permeable membrane. The dialysis fluid contains different salts in such a concentration that the waste products including the toxic substances are transferred from the blood of the patient into the dialysis fluid by diffusion and convection. The blood such cleaned from the waste products is fed back into the blood circulation of the patient via a blood outflow conduit connected to the dialyzer.
For quantification of the result of the kidney replacement therapy, it is necessary to control the efficiency of the kidney replacement therapy directly or online. Therefore, the so-called Kt/V model was developed. Therein, the Kt/V value is a parameter for estimation of the efficiency of a kidney replacement therapy, wherein the clearance K stands for the volume flow of the purified uremic substances, t for the time of treatment, and V for the volume of distribution of the patient. Thereby, K as well as V is related each to the particular waste product. Usually, the efficiency of a kidney replacement therapy is described using urea as a waste product, so that K describes the urea clearance and V the distribution volume of urea in the patient, which basically corresponds to the body water of the patient.
From EP1083948A1 and EP2005982A1, it is known to estimate the Kt/V value and the reduction rate RR, respectively, spectrophotometrically for a particular waste product during the kidney replacement therapy with the help of a measuring device located in the outflow Kt/V value using UV radiation and its absorption by substances obligatory excreted by urine in the dialysis fluid.
These known apparatus have been shown, however, that a consistent intensity of the radiation of the radiation source and a consistent sensitivity of the detector system could not be guaranteed neither over the operating time of the radiation source nor during a single kidney replacement therapy. Consequently, the absorption measurement in the used dialysis fluid during different treatments and also during a single treatment time is based on variable radiation intensities of the radiation source and/or a modified output signal for a constant input signal of the detection system. This implicates that the Kt/V value based on the absorption measurement and the reduction rate RR based on the absorption measurement, respectively, of a particular product are not corresponding to the real values. On the contrary, the absorption measurement in the spent dialysis fluid and, thus, the statement concerning the Kt/V value and the reduction rate RR, respectively, is falsified for a particular waste product.
Therefore, it is the problem to develop an apparatus according to the generic term of claim 1 in such a way that a reliable and genuine statement about the Kt/V value and the reduction rate RR, respectively, of a kidney replacement therapy is obtained by the absorption measurement.
A further problem is to provide a method for obtaining a reliable and genuine statement about the Kt/V value and the reduction rate RR, respectively, of a kidney replacement therapy.